Unlike the green plants, most bacteria are unable to obtain the energy that is required for their metabolism from sunlight. They must, therefore, make use of such chemical changes as will involve the release of energy.
As an example of the acquirement of energy in this way may be taken the oxidation of methane by B. methanicus. This organism, described by Söhngen, obtains its energy supply by the conversion of methane into CO2 and H2O.
CH4 + 2O2 = CO2 + 2H2O 220 Cal.
A further example is the acetic organism that obtains its energy through the oxidation of alcohol to acetic acid.
C2H6O + O2 = C2H4O2 + H2O 115 Cal.
The decomposition processes brought about by micro-organisms in obtaining energy are usually oxidations, but this is not necessarily so, as can be seen in case of the fermentation of sugar into alcohol.[E]
C6H12O6 = 2C2H6O + 2CO2 50 Cal.
[E] These examples are from Orla-Jensen (Centralblatt f. Bakt., II., Bd. 22, p. 305).
By far the greater part of the decomposition of organic matter is brought about by bacteria in the process of acquiring energy. In the soil, nearly the whole of the material utilised by bacteria as a source of energy is derived ultimately from green plants. The energy materials left in the soil by the plant fall into two groups, the non-nitrogenous compounds, which are mainly carbohydrates and their derivatives, and the nitrogenous compounds, principally derived from proteins.